It is known to provide surfaces of steel with a coating of metallic zinc by connecting the steel article as the cathode in a zinc-containing aqueous sulfuric acid electrolyte. After galvanic zinc coating the steel surfaces exhibit a much higher resistance to corrosion when exposed to various open-air atmospheric conditions.
It has been found that when an application of paint and the like coating materials is also intended it is desirable to provide the zinc surfaces with a phosphate coating before such a coating or painting by a treatment with an aqueous acid phosphatizing solution. After this treatment the resulting paint-zinc composite coating has a much higher bond strength and resistance to corrosion than zinc which has been painted without a phosphatizing pretreatment.
The phosphatizing baths used to treat the zinc contain zinc and phosphate as components which influence the formation of the coting. In addition, the coating may be modified by the presence of other cations, such as nickel, copper, calcium, manganese and alkali metal.
To accelerate the formation of the coating it is known to use oxidizing agents, e.g., of the group consisting of nitrates, chlorates, nitrites, peroxides, organic nitro compounds.
Other additives which may be used include, inter alia, fluorides, chlorides, organic polyhydroxycarboxylic acids, complex phosphates and surfactants. The baths are used with dipping, spraying-dipping and spraying operations at temperatures of usually 30.degree. to 70.degree. C. and with treatment times of, 0.5 to 5 minutes (EP-OS No. 69 950).
When it is desired to phosphatize surfaces of electrodeposited zinc which are contaminated with films of oil, grease and fat, the phosphatizing must be preceded by a degreasing treatment, which is usually carried out with an aqueous alkaline solution. A water rinse is used between the degreasing and phosphatizing treatments.
The phosphatizing treatment is succeeded by a water rinse, which is usually followed by a passivating afterrinse.
The uniform phosphate coating formed by the phosphatizing of zinc-coated steel surfaces is often found to have small whitish spots, which in a microscopic examination are found to consist of crystals accumulated at the rims of pits.
As phosphatizing proceeds, the zinc surface in contact with the phosphatizing solution is virtually completely coated with a phosphate coating but, for reasons not known thus far, several spots which are 0.1 to 1 mm in diameter are left uncoated and subjected to a continued pickling action by the phosphatizing solution.
Owing to the large amount of zinc ions present at the pits, tertiary zinc phosphate is precipitated at the rims of the pits and is built up like a rim of a crater. After painting, the spots appear as small elevations in the paint film and require expensive grinding operations for their removal.
A local corrosion of the zinc surface may also be caused during the cleansing and rinsing processes. Particularly the cleansing results in an efflorescence of crystals in an extent of a few millimeters in a direction that is parallel to the surface.
The formation of spots during the cleansing and during the phosphatizing may be inhibited by the use of treating solutions having special compositions, but the need for such special compositions has previously imposed a restriction of the range in which the composition of the treating solutions can be varied and has rendered more difficult the achieving of an optimum which would be desirable with a view to other objects, such as the improvement of the bond strength of the paint and the resistance to corrosion. For this reason there is a demand for zinc-coated surfaces which exhibit no or fewer spots.